Add --project-root flag to scip stats

[abitrolly]

Workaround for #98

This shares logic with bindings/go/scip/testutil/format.go, but I haven't found the best place to put this shared code.

Test plan

$ go build -o scip ./cmd && ./scip stats --from /tmp/index.scip --project-root /data/s/gdk
{
  "documents": 222,
  "linesOfCode": 14789,
  "occurrences": 11324,
  "definitions": 2908
}

$ go build -o scip ./cmd && ./scip stats --from /tmp/index.scip                          
2023/07/19 17:51:48 Project root [/s/gdk] doesn't exist, using current working directory [/home/anatoli/delme/scip] instead. To override this behaviour, specify --project-root=<folder> option
2023/07/19 17:51:48 error counting stats: lstat /home/anatoli/delme/scip/support/compare.rb: no such file or directory 

[olafurpg]

Thank you for this contribution! I just merged #189 which fixes the failing CI. Can you rebase on top of main? LGTM 👍🏻 once the CI is green

[abitrolly]

@olafurpg done!

[olafurpg]

Thank you!